SaaS Market Sizing Course

Most SaaS market numbers are wrong in a specific way. The arithmetic is fine. The market was never defined, so the number describes something nobody agreed on, and the first hard question in a board meeting takes it apart in about ninety seconds.

This course fixes the definition first and the spreadsheet second. Three lessons, roughly three hours, ending with a sourced model and a one page summary you can hand to an investor.

What the course produces

A bottom up model in a spreadsheet, with every input traced to a named source, and a one page summary that states the market definition in a sentence, the TAM, SAM and SOM figures, and the three assumptions the number is most sensitive to.

That last part is the one that wins meetings. Investors do not expect your number to be right. They expect you to know which input breaks it.

Lesson 2: build the bottom up model is the longest and does the real work. Lesson one is short and decides everything.

Why definition is the whole game

Take a compliance automation product. Is the market every company that needs SOC 2, every company that will need it in three years, every company already paying for a compliance tool, or every company above 20 employees selling to enterprise buyers? Those four definitions produce numbers that differ by more than 50x.

None of them is wrong. But only one matches who your sales team can actually reach this year, and choosing it is a positioning decision, not an arithmetic one. That is why sizing belongs to marketing.

Once the definition holds, total addressable market becomes a mechanical calculation rather than an argument.

What you need before lesson one

Three inputs, all of which you probably have.

A product definition specific enough to say what problem you solve and for whom. Your current pricing, including realistic average contract value rather than list price, because discounting is real and a model built on list price overstates by 15 to 30 percent. And a target segment hypothesis, even a rough one: industry, company size band, geography.

If you cannot state the segment hypothesis, stop and do positioning work first. Sizing an undefined market produces a number that changes every time someone asks about it.

The tools used throughout

The course runs on a spreadsheet, and the SaaS market sizing model template is the one we build in lesson two. Inputs sit on one tab, calculations on another, so the annual refresh is an edit rather than a rebuild.

Two calculators do the quick versions. The SaaS TAM calculator gives you a first pass in about two minutes if you already know your account count and ACV. The TAM SAM SOM calculator handles the narrowing from total market to what you can serve to what you can realistically win in three years. For the competitive half of the picture, the SaaS market share calculator turns a revenue figure into a share estimate, which is the number that usually appears on slide six.

Top down versus bottom up, settled

Build bottom up. Check top down. That is the whole rule, and the reason is that a bottom up model exposes its assumptions, which means it can be argued with rather than simply disbelieved.

Gartner and IDC figures are useful as the top down check and as context, but they measure a category boundary somebody else drew. Using one as your primary number means outsourcing the most important decision in the model. The top down versus bottom up comparison covers when the third approach is worth the effort.

A worked example you can follow

Lesson two carries one company through end to end: a $4M ARR compliance automation tool selling to North American B2B software companies above 30 employees. We count the accounts from a LinkedIn industry and headcount filter, cross check against a registry count, discount for companies already locked into a competitor, apply an ACV band rather than a single figure, and end with a range instead of a point estimate.

A range is the honest output. Anyone presenting a single number to the dollar is presenting false precision, and experienced investors read it as inexperience.

When to refresh, and what breaks

Refresh annually, and immediately after any material pricing change. Pricing flows straight into the ACV input, so a packaging change can move the headline number by 30 percent without anyone touching the account counts.

The other trigger is segment expansion. Adding a vertical or a geography changes both the count and the ACV band, and it is usually the moment somebody notices the original definition excluded a segment you have been selling to for six months.

Before you present, read a few real decks. The SaaS TAM slide teardowns collection shows which framings survived diligence and which ones drew the question that ended the meeting.

Start the course

Enter your email above for the worksheet bundle: the market definition sheet, the sourcing checklist with the account count sources ranked by bias, and the model template. Lessons arrive every three days.

Then do lesson one properly. If you only have an hour this week, spend it on the definition rather than the spreadsheet, because a clean definition makes the arithmetic almost automatic and a vague one makes it worthless. The wider context on category size and growth sits in the SaaS market size and growth hub.